This invention relates to the cooling of crude gas mixtures containing hydrocarbons of a wide boiling point range, and especially to the cooling of cracked hydrocarbon gases, particularly to thermally cracked hydrocarbons.
In the thermal cracking of hydrocarbons, for example, a mixture predominating paraffinic hydrocarbons of 2-5 carbon atoms, a cracked gas is obtained at high temperatures, e.g., between 800.degree. and 1000.degree. C., said cracked gas consisting essentially of a broad spectrum of hydrocarbons having a wide boiling point range. References incorporated herein for a detailed description of thermal cracking are Ullmanns Encyklopaedie der technischen Chemie 4th edition, Volume b 1974, page 158 - 194, Verlag Chemie GmbH S. B. Edomik, E. J. Greem, C. P. Hallee: Manufacturing Ethylene, The Petroleum Publishing Co., 211 South Cheyenne, Tulsa, USA, reprinted from Oil Gas J. 1966 - 1970.
To avoid secondary reactions, the high temperature cracked gas is first quenched in so-called quenching or cracked-gas coolers to an intermediate temperature of about 250.degree. to 400.degree. C., for example about 350.degree. C. Further cooling is not desirable since heavy hydrocarbon fractions are condensed which tend to polymerize and to form coke at the tube surface of the quench coolers. Therefore the cooling is limited to the temperatures stated above in order to obtain an economical heater and quench cooler run length.
The further cooling of this crude gas mixture, containing primarily hydrocarbons of 1-8 carbon atoms but also being contaminated with minor amounts of higher-boiling hydrocarbons, as well as coke and tar, to approximately ambient temperature is normally carried out in direct heat exchange with water in a suitable column. At the sump of such column there is obtained a mixture of water, liquid hydrocarbons, as well as tar and coke, and the mixture is then subjected to a water-separating step in a gravity-activated separator, e.g. a decanter. The thus-obtained water is recooled in one or more heat exchangers in indirect heat exchange with a refrigerant and then recycled to the column. A disadvantage of the above described cooling technique for the quenched gas is that the hydrocarbons cannot be completely separated from the water in the gravity-operated separator. As a consequence, the residual unseparated solid and liquid hydrocarbons in the water essentially cause the heat exchangers in the cooling cycle to be fouled and/or clogged. As a consequence, the heat exchangers must be frequently cleaned on the one hand, and the system must be provided with backup heat exchangers on the other hand. This results in increased operating and investment costs.